668 research outputs found
Constraints on neutron star radii based on chiral effective field theory interactions
We show that microscopic calculations based on chiral effective field theory
interactions constrain the properties of neutron-rich matter below nuclear
densities to a much higher degree than is reflected in commonly used equations
of state. Combined with observed neutron star masses, our results lead to a
radius R = 9.7 - 13.9 km for a 1.4 M_{solar} star, where the theoretical range
is due, in about equal amounts, to uncertainties in many-body forces and to the
extrapolation to high densities.Comment: 4 pages, 4 figures; NORDITA-2010-4
Nuclear symmetry energy and core-crust transition in neutron stars: a critical study
The slope of the nuclear symmetry energy at saturation density is pointed
out as a crucial quantity to determine the mass and width of neutron-star
crusts. This letter clarifies the relation between and the core-crust
transition. We confirm that the transition density is soundly correlated with
despite differences between models, and we propose a clear understanding of
this correlation based on a generalised liquid drop model. Using a large number
of nuclear models, we evaluate the dispersion affecting the correlation between
the transition pressure and . From a detailed analysis it is shown
that this correlation is weak due to a cancellation between different terms.
The correlation between the isovector coefficients and plays
a crucial role in this discussion
Emergency and on-demand health care: modelling a large complex system
This paper describes how system dynamics was used as a central part of a whole-system review of emergency and on-demand health care in Nottingham, England. Based on interviews with 30 key individuals across health and social care, a 'conceptual map' of the system was developed, showing potential patient pathways through the system. This was used to construct a stock-flow model, populated with current activity data, in order to simulate patient flows and to identify system bottle-necks. Without intervention, assuming current trends continue, Nottingham hospitals are unlikely to reach elective admission targets or achieve the government target of 82% bed occupancy. Admissions from general practice had the greatest influence on occupancy rates. Preventing a small number of emergency admissions in elderly patients showed a substantial effect, reducing bed occupancy by 1% per annum over 5 years. Modelling indicated a range of undesirable outcomes associated with continued growth in demand for emergency care, but also considerable potential to intervene to alleviate these problems, in particular by increasing the care options available in the community
The Equation of State of Neutron-Star Matter in Strong Magnetic Fields
We study the effects of very strong magnetic fields on the equation of state
(EOS) in multicomponent, interacting matter by developing a covariant
description for the inclusion of the anomalous magnetic moments of nucleons.
For the description of neutron star matter, we employ a field-theoretical
approach which permits the study of several models which differ in their
behavior at high density. Effects of Landau quantization in ultra-strong
magnetic fields ( Gauss) lead to a reduction in the electron
chemical potential and a substantial increase in the proton fraction. We find
the generic result for Gauss that the softening of the EOS caused
by Landau quantization is overwhelmed by stiffening due to the incorporation of
the anomalous magnetic moments of the nucleons. In addition, the neutrons
become completely spin polarized. The inclusion of ultra-strong magnetic fields
leads to a dramatic increase in the proton fraction, with consequences for the
direct Urca process and neutron star cooling. The magnetization of the matter
never appears to become very large, as the value of never deviates from
unity by more than a few percent. Our findings have implications for the
structure of neutron stars in the presence of large frozen-in magnetic fields.Comment: 40 pages, 7 figures, accepted for publication in Ap
Isospin-rich nuclei in neutron star matter
Stability of nuclei beyond the drip lines in the presence of an enveloping
gas of nucleons and electrons, as prevailing in the inner crust of a neutron
star, is studied in the temperature-dependent Thomas-Fermi framework. A
limiting asymmetry in the isospin space beyond which nuclei cannot exist
emerges from the calculations. The ambient conditions like temperature, baryon
density and neutrino concentration under which these exotic nuclear systems can
be formed are studied in some detail.Comment: Submitted to Phy. Rev. C: Revtex version of manuscript 22 pages and
10 PS-files for figure
Nuclei beyond the drip line
In a Thomas-Fermi model, calculations are presented for nuclei beyond the
nuclear drip line at zero temperature. These nuclei are in equilibrium by the
presence of an external gas, as may be envisaged in the astrophysical scenario.
We find that there is a limiting asymmetry beyond which these nuclei can no
longer be made stable.Comment: Physical Review C (in press), 1 ReVteX file for text, 4 PS-files for
figure
Isospin-rich nuclei in neutron star matter
Stability of nuclei beyond the drip lines in the presence of an enveloping
gas of nucleons and electrons, as prevailing in the inner crust of a neutron
star, is studied in the temperature-dependent Thomas-Fermi framework. A
limiting asymmetry in the isospin space beyond which nuclei cannot exist
emerges from the calculations. The ambient conditions like temperature, baryon
density and neutrino concentration under which these exotic nuclear systems can
be formed are studied in some detail.Comment: Submitted to Phy. Rev. C: Revtex version of manuscript 22 pages and
10 PS-files for figure
Neutron Star Structure and the Neutron Radius of 208Pb
We study relationships between the neutron-rich skin of a heavy nucleus and
the properties of neutron-star crusts. Relativistic effective field theories
with a thicker neutron skin in Pb have a larger electron fraction and a
lower liquid-to-solid transition density for neutron-rich matter. These
properties are determined by the density dependence of the symmetry energy
which we vary by adding nonlinear couplings between isoscalar and isovector
mesons. An accurate measurement of the neutron radius in Pb---via
parity violating electron scattering---may have important implications for the
structure of neutron stars.Comment: 5 pages 3 figures, added additional evidence of model independence,
Phys. Rev. Letters in pres
Evolution of Protoneutron Stars
We study the thermal and chemical evolution during the Kelvin-Helmholtz phase
of the birth of a neutron star, employing neutrino opacities that are
consistently calculated with the underlying equation of state (EOS).
Expressions for the diffusion coefficients appropriate for general relativistic
neutrino transport in the equilibrium diffusion approximation are derived. The
diffusion coefficients are evaluated using a field-theoretical finite
temperature EOS that includes the possible presence of hyperons. The variation
of the diffusion coefficients is studied as a function of EOS and compositional
parameters. We present results from numerical simulations of protoneutron star
cooling for internal stellar properties as well as emitted neutrino energies
and luminosities. We discuss the influence of the initial stellar model, the
total mass, the underlying EOS, and the addition of hyperons on the evolution
of the protoneutron star and upon the expected signal in terrestrial detectors.Comment: 67 pages, 25 figure
Accelerating incoherent dedispersion
Incoherent dedispersion is a computationally intensive problem that appears
frequently in pulsar and transient astronomy. For current and future transient
pipelines, dedispersion can dominate the total execution time, meaning its
computational speed acts as a constraint on the quality and quantity of science
results. It is thus critical that the algorithm be able to take advantage of
trends in commodity computing hardware. With this goal in mind, we present
analysis of the 'direct', 'tree' and 'sub-band' dedispersion algorithms with
respect to their potential for efficient execution on modern graphics
processing units (GPUs). We find all three to be excellent candidates, and
proceed to describe implementations in C for CUDA using insight gained from the
analysis. Using recent CPU and GPU hardware, the transition to the GPU provides
a speed-up of 9x for the direct algorithm when compared to an optimised
quad-core CPU code. For realistic recent survey parameters, these speeds are
high enough that further optimisation is unnecessary to achieve real-time
processing. Where further speed-ups are desirable, we find that the tree and
sub-band algorithms are able to provide 3-7x better performance at the cost of
certain smearing, memory consumption and development time trade-offs. We finish
with a discussion of the implications of these results for future transient
surveys. Our GPU dedispersion code is publicly available as a C library at:
http://dedisp.googlecode.com/Comment: 15 pages, 4 figures, 2 tables, accepted for publication in MNRA
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